Heart Impulse Block in Cats

Sinus Arrest and Sinoatrial Block in Cats

The sinoatrial node (SA Node, or SAN), also called the sinus node, is the initiator of electrical impulses within the heart, triggering the heart to beat, or contract, by firing off electrical surges. Sinus arrest is a disorder of heart beat impulse formation caused by a slowing down or cessation of spontaneous sinus nodal automaticity – the automatic behavior of the tissues that set the pace for the heart's rhythm. It is the failure of the sinoatrial (SA) node to initiate an impulse at the expected time that leads to sinus arrest. Persistent sinus arrest that is not due to the use of a drug is often indicative of sick sinus syndrome (SSS) - a disorder of the heart’s electrical impulse formation within the sinus node.

Sinoatrial block is a disorder of impulse conduction. This is when an impulse formed within the sinus node fails to be conducted through the atria (the interior of the heart), or when it does so with delay. Most commonly, the basic rhythm of the sinus node is not disturbed when the impulses fail to conduct properly.

Symptoms and Types

Usually asymptomatic (without symptoms)

Weakness

Fainting

Pale gums

Very slow heart rate, may be possible to detect

Sinoatrial block is classified into first, second, and third-degree SA block (similar to degrees of atrioventricular [AV] block). It is difficult to diagnose first and third-degree SA block from an electrocardiogram (ECG) reading only.

Second-degree SA block is the most common type of SA block, and the only degree that can be recognized on a surface ECG. In addition, there are two types of second-degree SA blocks: Mobitz type I (also called Wenckebach periodicity) and Mobitz type II.

Diagnosis

Your veterinarian will perform a complete physical exam on your cat, with a blood chemical profile, a complete blood count, an electrolyte panel and a urinalysis. The electrolyte panel may show hyperkalemia, abnormal levels of potassium in the blood, which can lead to arrhythmias. You will need to give a thorough history of your cat's health, including a history of symptoms, and their onset.

Thoracic (chest) x-rays and/or a cardiac ultrasound imaging may be taken by your veterinarian to confirm or rule out heart disease and abnormal tissue growth (neoplasia).

A provocative atropine response test may be done to assess sinus node function. This test uses the drug atropine to stimulate the firing action of the SA Node. Cats with SSS generally will have no response, or will have an incomplete response to the atropine.

Treatment

Most patients will be treated on an outpatient basis. Only patients showing clinical signs of illness should be hospitalized. Fluid therapy will be given to patients in need of it. Very ill patients that are not responding to medical therapy may require implantation of an artificial pacemaker, and will be hospitalized prior to surgery in preparation of it. If your pet becomes excessively weak, or is showing signs of losing consciousness, or fainting, it will need to have its activity restricted.

Living and Management

After care will be dependent on whether your cat has an underlying disease, along with the SA block. Your veterinarian will schedule follow-up appointments as necessary, and an ECG reading will be done at each visit to follow your cat's progress. If your cat becomes weak, or loses consciousness, contact your veterinarian immediately for advisement.

pharynx

A cavity in the mouth where the respiratory systems and gastrointestinal systems come together

sinoatrial node

A lump of tissue inside the right atrium; it helps to regulate the beat of the heart

urinalysis

An in-depth examination of the properties of urine; used to determine the presence or absence of illness

nerve

A bundle of fibers that are used in the process of sending impulses through the body

sinus

A cavity within a bone; may also indicate a flow or channel

implantation

An attachment of the zygote inside the uterus

asymptomatic

Term used to refer to a condition of having a disease or affliction but not displaying symptoms of it.